1. Field of the Invention
The present invention relates to a semiconductor process for fabricating integrated circuit devices. More specifically, the present invention relates to a process used to form a tungsten plug in a contact opening or via.
2. Discussion of Related Art
Tungsten (W) plugs are commonly used to provide contact to the upper surfaces to active regions of circuits formed in a semiconductor wafer. To help the tungsten adhere to the semiconductor surface, an adhesion layer, such as titanium nitride (TiN) is commonly deposited before the tungsten. The adhesion layer readily adheres to the semiconductor surface, and provides a layer to which the tungsten will readily adhere.
FIG. 1A is a cross-sectional view of a conventional semiconductor structure 100 that includes semiconductor substrate (wafer) 10, source region 20, drain region 21, gate structure 22 and field oxide layer 23. Regions 20A and 21A are contact regions where contact is to be made to source 20 and drain 21, respectively, of the MOS transistor. Dielectric layer 24 is formed over the upper surface of the MOS transistor structure. Contact openings 25 are formed through dielectric layer 24 to expose contact regions 20A and 21A. An adhesion layer 31, which includes a layer of TiN, is formed over the resulting structure, and extends into openings 25. A tungsten (W) contact layer 32 is formed over adhesion layer 31, and also extends into openings 25.
Tungsten layer 32 is etched back using a blanket plasma etch process. It is desirable for this plasma etch process to stop on adhesion layer 31, thereby forming tungsten plugs 32P (FIG. 1B). However, if the plasma etch inadvertently removes a portion of adhesion layer 31, then the plasma etch will rapidly remove the underlying portion of dielectric layer 24 (and field oxide layer 23, if present under the opening in adhesion layer 31).
FIG. 1B illustrates an opening 41, which is inadvertently formed through adhesion layer 31 during the plasma etch. Opening 41 extends through adhesion layer 31 into the underlying dielectric layer 24. Note that the plasma etch also forms tungsten plugs 32P from tungsten layer 32.
As shown in FIG. 1C, a first conductive layer 51, such as aluminum, is subsequently deposited over the upper surface of the resulting structure. First conductive layer 51 contacts tungsten plugs 32P. A portion 51A of first conductive layer 51 fills opening 41.
As shown in FIG. 1D, a patterned mask 61 is formed over the upper surface of first conductive layer 51. Mask 61 includes an opening 62, which exposes a portion of the first metal layer to be etched. An etch is performed through opening 62, with the intent of etching entirely through first conductive layer 51, thereby forming two separate conductive traces. However, it is difficult for the etch to entirely remove aluminum portion 51A from opening 41. As a result, part of aluminum portion 51A may remain in opening 41 at the end of the etch. As illustrated in FIG. 1D, any remaining part of aluminum portion 51A may provide an undesirable short between the intended separate conductive traces.
One method of preventing etch-through of the adhesion layer is described in U.S. Pat. No. 5,804,502 to Gabriel et al., which suggests providing a thicker adhesion layer near the edges of the semiconductor structure. While etching the tungsten layer, the semiconductor structure of Gabriel et al. becomes hotter near the edges than at the center. The higher temperature at the edges results in faster etching of the tungsten and adhesion layers near the edges. Gabriel et al. therefore provides an adhesion layer that is thicker near the edges, thereby preventing all of the adhesion layer from being removed at the edges of the wafer.
Another method for improving the etching of a tungsten layer and an adhesion layer is described in U.S. Pat. No. 5,915,202 to Lo et al., which suggests performing a two-step etchback process. In the two-step etchback process of Lo et al., the first etch takes place in a first RF plasma in a first flowing gas mixture of oxygen, argon and SF6, and the second etch takes place in a second flowing gas mixture of oxygen and an inert gas. Lo et al. teaches that it is critical to sustain uninterrupted RF power between the first and second steps (preventing the substrate from cooling) in order to reduce etch byproduct redeposition onto the plasma chamber walls and other plasma etch apparatus features.
It would therefore be desirable to have a method for performing an etchback of a tungsten layer that overlies an adhesion layer, without etching through the adhesion layer, thereby avoiding the above-described problems.